In association with the spread of the Internet, a collaboration system in which some object is shared between remote places has widely been used. In the conventional collaboration system, a server/client type is a mainstream. A server makes management and distribution of data as a target for collaboration. Since the server unitarily makes the management of the data, the data for the collaboration possessed by each client is held identical.
However, since a load on the server is large in the collaboration system of the server/client type, in recent years, attention has been paid to a collaboration system of a peer-to-peer type. In the peer-to-peer type, each user participating in the network is equal (Peer) and does not have a special server. A user apparatus which is used in the peer-to-peer type network is simply called a peer in the following description.
However, in such a conventional collaboration system of the peer-to-peer type, since the data for the collaboration is held in each user, there is such a problem that in the state where a specific peer does not participate in the collaboration, in the case where the residual peers update the data or the like, consistency of the data in each peer cannot be held. In the case of the server/client type, the server serving as a distributing source of the data generally can be set at a place of an excellent network environment. However, in the case of the peer-to-peer type, a distributing source of the data is each peer and there are various environments such as wireless LAN, telephone line, and the like.
There is, consequently, such a problem that in the case where the data is distributed from a transmitting peer serving as a start point to all of the other peers at a ratio of 1:n, the data cannot be efficiently transmitted in dependence on a distribution path, it takes a long communication time, and communication costs are high.